Recently, the need for the improvement of photographic performance of silver halide light sensitive photographic materials has also become severe, and further higher level requirements have also been made for photographic performance including high speed, superior graininess and improved process stability.
U.S. Pat. No. 3,726,681 discloses a technique for improving graininess, in which a coupler having a higher coupling rate is employed in a high speed emulsion layer and a coupler having a lower coupling rate is employed in a low speed emulsion layer, thereby leading to higher sensitivity and improved graininess.
JP-A 59-60437 (herein, the term, JP-A means unexamined and published Japanese Patent Application) discloses a technique for enhancing graininess and sharpness, in which a highly reactive coupler is contained in the highest speed emulsion layer and in at least one of other layers with the same spectral sensitivity is employed a lower reactive coupler and a DIR compound capable of releasing a diffusible development inhibitor or a diffusible development retarding precursor.
JP-A 2-259754 discloses a technique for improving sharpness in which among at least three emulsion layers with the same spectral sensitivity, a coupler having a lower coupling rate is employed in a medium speed layer and a DIR compound is employed in a high speed layer.
However, these techniques are insufficient in stability with respect to processing fluctuations, and further improvements are desired in terms of higher sensitivity and superior graininess.
In addition, the need for improvement of silver halide emulsions has also become marked, and further higher level requirements have also been made for photographic performance including high speed, superior graininess and improved storage stability.
There are known a variety of methods for enhancing the sensitivity of silver halide emulsions, including the method for preparing a silver halide emulsion, a chemical sensitization technique of silver halide emulsions, a spectral sensitization technique of silver halide emulsions, the method for designing a silver halide photographic material and the processing method of silver halide photographic materials. Of these, the most preferable and essential method concerns enhancement of the photographic quantum yield of silver halide grains.
The well known technique for enhancing the quantum yield in the latent image forming process of silver halide grains is employing a core having a high silver iodide content within the grain, and JP-A 63-92942 discloses a technique of providing a core with a high silver iodide content in the interior of a tabular grain. JP-A 7-92594 also discloses a technique of silver halide grains comprising a core with a high silver iodide content and a low silver iodide containing portion localized in the vicinity of the grain surface.
However, it was proved that these grains had problems such that they were fast in developing speed, resulting in deterioration of graininess.